Efficient Nitrogen-Fixing Bacteria Isolated from Soybean Nodules in the Semi-arid Region of Northeast Brazil are Classified as Bradyrhizobium brasilense (Symbiovar Sojae)

Soybean ( Glycine max L.) is an important legume that greatly benefits from inoculation with nitrogen-fixing bacteria. In a previous study, five efficient nitrogen-fixing bacterial strains, isolated from nodules of soybean inoculated with soil from semi-arid region, Northeast Brazil, were identified as a new group within the genus Bradyrhizobium . The taxonomic status of these strains was evaluated in this study. The phylogenetic analysis of the 16S rRNA gene showed the high similarity of the five strains to Bradyrhizobium brasilense UFLA03-321 T (100%), B . pachyrhizi PAC48 T (100%), B. ripae WR4 T (100%), B . elkanii USDA 76 T (99.91%), and B. macuxiense BR 10303 T (99.91%). However, multilocus sequence analysis of the housekeeping genes atpD , dnaK , gyrB , recA, and rpoB , average nucleotide identity, and digital DNA–DNA hybridization analyses supported the classification of the group as B. brasilense. Some phenotypic characteristics allowed differentiating the five strains and the type strain of B. brasilense from the two neighboring species ( B. pachyrhizi PAC48 T and B. elkanii USDA 76 T ). The nodC and nifH genes’ analyses showed that these strains belong to symbiovar sojae, together with B. elkanii (USDA 76 T ) and B. ferriligni (CCBAU 51502 T ). The present results support the classification of these five strains as Bradyrhizobium brasilense (symbiovar sojae).